The manufacture of mineral fiber thermal insulation is carried out according to one of a number of continuous processes werein the molten mineral material flowing from a melting furnace is divided into streams and attenuated into fibers. The fibers are collected in a felted haphazard manner to form a mat. In order to produce most thermal insulating products, the fibers must be bonded together in an integral structure. Thus, thermal insulation batts, acoustical tiles and similar structures, generally comprising glass wool, rock wool, or other mineral fibers, are held together by a resinous binder present in amounts of about 5-20% by weight to impart strength and resiliency to the structure and preserve original insulating, acoustical, dimensional and other properties.
To achieve this binding, a curable binder material is added to the mineral wool. The curable binder composition is generally formulated as a liquid to be sprayed onto the fibers as they are dropping onto the collecting conveyor belt, in the process of forming the mat. The layer of fiber with binder is then compressed and shaped into the form and dimensions of the desired thermal insulating product and passed through a curing oven.
Various materials have been used as binder ingredients including both organic and inorganic materials. The organic binder materials most commonly employed are heat-curable thermosetting resin systems of the phenol formaldehyde type.
Such binders are generally provided as water soluble or water dispersable compositions which can be easily blended with other ingredients and diluted to low concentrations which are readily sprayed onto the fiber as it falls onto the collecting conveyor. The binder composition is often applied in an amount such that the cured binder constitutes about 5% to about 10% by weight of the finished product, though it can be as little as 1% or less or as high as 20% or more, depending upon the type of fiber product. Optimally, the amount of binder for most thermal insulating products will be the amount necessary to lock each fiber into the mass by bonding the fibers where they cross or overlap. For this reason, the binder compositions should have good flow characteristics, so that the binder solution can be applied at a low volume that will flow to the fiber intersections.
Generally, the binder system requirements are for a low-cost water soluble or water dispersable composition which can be easily applied and readily cured during normal production cycles. The product should be relatively stable for periods of time long enough to permit mixing and application at temperatures ordinarily encountered in fiberizing plants, and the composition should be dilutable in order to permit variations in concentrations for different end products. The cured binder product must provide a strong bond with sufficient elasticity and thickness recovery to permit reasonable shipping and in-service deformation of the thermal insulating product. It should be moisture resistant so that it will not settle under humid conditions. The most important test of humidity resistance is thickness recovery upon the opening of a package of insulation that has been tightly wound under compression for packaging, shipping, and storage. After prolonged storage, such as for two weeks to 12 months in a warm and humid environment, such as may be found in Tampa, Fla., or in Washington, D.C. during the summer months, thickness recovery is important if full insulating value is to be obtained after installation. It should be odor free and non-corrosive to metals with which it comes in contact.
Typical binder compositions are prepared as aqueous solutions or dispersions of partially condensed phenolic formaldehyde resins to which are added a variety of modifiers or agents to improve the spraying, flowing or similar application characteristics as well as the bonding strength, temperature resistance and other in-service features of the cured resin.
A binder composition much desired is one that can withstand service temperatures of 700.degree. F. to 900.degree. F. Numerous attempts have been made to provide such high temperature-resistant binders with little acceptance for practical commercial utility. A particularly troublesome aspect of binders prepared from phenol formaldehyde resin systems is the tendency towards punking when subjected to temperatures in excess of 600.degree. F., which are within the range of temperatures that may be encountered in use. The phenol formaldehyde resin system in the final thermoset stages can undergo exothermic oxidation. Since the mineral fiber product is an excellent heat insulator, the heat given off by any exothermic reaction in the binder is confined and becomes cumulative until temperatures are attained which cause thermal decomposition of the organic binder materials. In some instances, the decomposition products will have an ignition temperature lower than the corresponding temperature of the surrounding composition and may burst into flame.
The characteristic of exothermic decomposition without flame is referred to as "punking". It can continue at a slow rate for a considerable length of time, ultimately resulting in total decomposition of the binder with consequent deterioration of the fiberglass insulating product. Punking can be initiated in the cured phenolic binder systems by subjecting the thermal insulating product to elevated temperatures. There are very few mineral fiber thermal insulations produced with phenolic resin compositions that are commercially available at the present time which can withstand temperatures in excess of about 600.degree. F. to 700.degree. F., without punking, though numerous attempts have been made to formulate suitable binder compositions which can withstand high temperatures, since generally glass fibers can withstand temperatures up to 1200.degree. F. to 1300.degree. F. before softening. In fact, the glass fiber most commonly used in thermal insulating products at present has a Littleton softening point between 1250.degree. F. and 1300.degree. F. Accordingly, there is a definite need for a high temperature resistant binder which will extend the use of these glass fiber materials to high temperature service.
In the past, a variety of approaches have been taken in an effort to formulate high temperature binder compositions using phenolic resins as the primary binder ingredient. Modification of the phenolic formaldehyde by the addition of nitrogenous modifiers such as dicyandiamide is disclosed in U.S. Pat. No. 3,223,668. Others have tried the use of borates of various sorts as additions to the binder composition; U.S. Pat. Nos. 2,931,738, 3,002,857, 3,253,948, and 3,839,236.
There is also a need for phenolic resin binder materials that are water dilutable, non-punking, and that are stable under the refrigerated storage conditions often encountered in industrial situations.
As is pointed out in U.S. Pat. No. 3,072,595, urea has been used in the past in phenolic resin binders, primarily for its contribution of non-punking properties. Such non-punking binder compositions have heretofore been obtained chiefly by the use, in conjunction with a water soluble phenolic resin, of a urea or a urea-formaldehyde resin in sufficient quantity to provide about 15% by weight nitrogen based on total resin solids in the binder composition. These nonpunking compositions have been produced in various ways, namely simultaneous reaction of phenol, urea, and formaldehyde; reaction of phenol alcohols and urea; reaction of phenols and methylol ureas; and combinations of the foregoing. The cost of these non-punking resins, however, in terms of processing difficulties, erratic storage behavior, and precuring tendencies has made these urea-containing resins commercially unacceptable. The difficulties associated with these resins are, in general, attributable to the relatively great reactivity of urea.
The result of the presence of urea resins in previous non-punking insulating batt binders has been a marked lack of stability upon water dilution and a tendency to precure during batt manufacture.
In order for a resin to be commercially acceptable as a binder in insulation, the resin must have stability, i.e., show no signs of phase separation or the formation of hard, paste-like insoluble phases. Binders containing urea resin are often prone to undergo irreversible phase separation, and this tendency is increased when the resin is diluted with water. Frequently, this separation occurs in as few as five or six hours, and hence, prior art binders containing urea or urea resins are extremely disadvantageous from a processing standpoint and undesirable despite their non-punking character.
As described above, the binder is sprayed onto the molten material, cooling and coating the material to form coated fibers which form a mat. It has been found that binders containing urea resins are often either deficient in their degree of bond strength, or have a tendency to punk. This is because during the water/liquid binder spraying step the urea resin precures. As a result, the proportion of binder which actually is available for bonding is reduced. Unless the amount of binder is increased, the quality of the bond obtained is inferior. Increasing the binder in the batt increases the tendency to punk, since punking is roughly proportionate to binder content.
Additionally, the presence of urea resins in the batt bond may adversely affect water resistance. Cured urea-formaldehyde resins generally have poor humidity resistance and water resistance. Cured binder systems having a urea-formaldehyde resin content are, to the extent of that content, usually vulnerable to attack by water. When a binder mixture comprising a urea-formaldehyde resin and either a phenol or a phenolic resin is cured, little or no inter-reaction occurs between the urea resin and the phenol or the phenolic resin. This is because the reactivity of a urea resin molecule towards any other urea or urea resin molecules present is much greater than its reactivity towards a phenol or a phenolic resin. As a result, urea self-reaction or "homopolymerization" proceeds so quickly as to virtually exclude urea resin-phenol or urea resin-phenolic resin inter-reaction or "copolymerization". The cured product, therefore, is a mixture predominantly containing cured urea resin (homopolymer) and phenol, or cured phenolic resin (homopolymer) and very little, quite possibly no, urea-phenolic resin inter-reaction product (copolymer).
The use of urea solids in liquid phenolic resoles for inorganic or glass fiber bonding is known in the art and has achieved widespread commercial acceptance because:
(i) Urea imparts antipunk character to the resole.
(ii) Urea acts as a binder extender since it readily combines with the free formaldehyde in the resin under mildly alkaline or neutral pH conditions forming in situ methylol urea, a material that polymerizes readily under the curing conditions used in the fiber bonding process, forming urea formaldehyde adhesive.
(iii) Urea reduces the free formaldehyde content of the resole, thereby significantly reducing the release of formaldehyde from the resole, and the release of other volatiles as well. This reduction in volatiles increases retention of the binder on the hot glass fiber during application and during the curing process, and therefore improves efficiency.
(iv) Urea is presently approximately one-fifth the cost of solid phenolic resole and therefore acts as an economic extender.
The prior art teaches that urea should be added to the advanced resole shortly before the application of the liquid resole to the glass fibers to be bonded. This is necessary because when urea is added, methylol urea is immediately formed by the reaction of the urea with the free formaldehyde that is present in the resole solution. The methylol urea in turn polymerizes, causing a rapid loss of resin solubility in water. High resin solubility in water is necessary since dilution water must be added to the binder to cool the hot glass fibers during application and thereby prevent precuring of the resin. The dilution water is also used because it facilitates the even distribution of the relatively small amount of binder solids onto a large amount of glass fiber, and it also facilitates the desired flow of the resin.
It is possible to severely limit the polymerization reaction of methylol urea by storing the resulting methylol urea resole solution under refrigerated conditions; however, this results in the formation of precipitates of tetramethylol 4-4' dihydroxydiphenyl methane (tetra dimer) when the free formaldehyde content is below 2% of the solution.
The preparation of resoles containing 2-2' and 2-4' dihydroxydiphenyl methanes is described in the Higginbottom patents, U.S. Pat. Nos. 3,956,204, 3,956,205 and 4,060,504, and Canadian Patent No. 1,049,172. The preparation of resoles containing methyoxymethyl aminotriazines or methoxymethyl melamines is described in Canadian Patent No. 1,026,882, to Higginbottom. These compositions were developed for the purpose of inhibiting tetra dimer crystallization at low temperatures. These preparations have not achieved commercial acceptance because the resulting resoles are not fully soluble in water and require the addition of expensive proteinaceous emulsifiers to prevent the formation of sticky, gummy substances that plug application equipment when the resole solution is diluted with 10 to 20 parts of water prior to application.
In addition, the production of emulsifiable resoles normally requires an additional ingredient, commonly oxalic acid or sulfuric acid, to precipitate and thereby inactivate the commonly present divalent metal ion in the resole that would otherwise precipitate the proteinaceous emulsifier that is added to emulsify the resole upon dilution. The divalent ion is ordinarily an alkaline earth metal such as calcium, barium, or magnesium, whose oxide provides the necessary alkalinity to catalyze the formation of the resole from phenol and formaldehyde.
It is also known in the art to prepare a green low advanced resole where the tetramethylol 4-4' dihydroxydiphenyl methane (tetra dimer) concentration does not exceed more than about 5% of the total phenol originally present. Under these conditions, tetra dimer does not crystallize from solution at low temperature when urea is added to the resole. These conditions, however, necessitate that the resole contain large amounts of free phenol and monomethylol phenols since the reaction must be stopped normally by cooling and neutralization with acid before the reaction with phenol and formaldehyde is complete.
Continued high temperatures and alkalinity cause rapid condensation of the relatively high concentration of trimethylol phenol to tetra dimer. High concentrations of free phenol and of monomethylol phenol in the resole are undesirable because these materials are relatively volatile and vaporize readily in the hot air curing process after application of the solution to the hot glass fiber. Also, the free phenol and the monomethylol phenols are relatively unreactive towards polymerization with remaining methylol phenols and also are relatively unreactive towards copolymerization with the methylol ureas. This causes poor binder efficiency and performance.
The state of the art therefore makes it necessary for the manufacturers of glass fiber thermal insulation, who use resole resins, to add urea to the resole shortly before application to the glass fiber. This in turn necessitates maintaining an inventory of urea, and in many cases, also providing for expensive solids handling equipment for bulk urea. In addition, these manufacturers must also provide additional mixing equipment for combining the solid urea with the liquid resole prior to application to the glass fiber. Manufacturers of glass fiber products also find they must deal with the formaldehyde vapors associated with handling ordinary resoles which are most objectionable.
For economical, practical environmental and industrial hygenic reasons, resole urea compositions that are storage stable, fully soluble in water, and very low in free phenol and free formaldehyde content, have therefore been long sought-after by the phenol/formaldehyde, urea/formaldehyde resin adhesive manufacturing industry.